body {
    background-image: url("Imagenes/fondo.jpg");
    background-position:center;
    background-repeat:no-repeat;
    background-attachment:fixed;
    background-size:cover;
    color:white;
    text-align:center;
}

.centrado-porcentual {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
    -webkit-transform: translate(-50%, -50%);
}